The turbulent past of the black hole in our Milky Way

12/07/2010

The supermassive black hole at the centre of our Galaxy has lived through eventful times during the last few centuries. The secret of its turbulent past has been revealed by its immediate surroundings. The variable X-ray and gamma ray luminosity of the molecular clouds point to a major flare in the recent past. These results, obtained by an international group of researchers under the leadership of French astrophysicists have been published in The Astrophysical Journal.Black holes in active galactic nuclei are usually intensely active. But not in our Galaxy. Our black hole, Sgr A* is extremely quiet. However, an international team of researchers led by astrophysicists from the Astroparticle and Cosmology Laboratory in Paris (CNRS / Paris-Diderot University / CEA-IRFU / Paris Observatory), has succeeded in showing that this black hole was active in the very recent past, and that it is not so atypical as it first appeared.

Using the ESA XMM-Newton and INTEGRAL satellites, built with significant contributions from the CNES, the CEA and CNRS-INSU, the researchers observed X-ray and gamma ray emissions from the molecular clouds in the region of the galactic centre and close to Sgr A*. The results were surprising; these emissions have been subject to spectacular variations, including the gradual brightening of one of the clouds between 2004 and 2009 at an apparent speed three times greater than the speed of light (Fig. 1).

Figure 1. XMM-Newton images of the neutral iron fluorescence line in the molecular clouds surrounding Sgr A* (Bridge, MC1 and MC2) between 2004 and 2009. The progression of the emission in the sub-regions indicated by ellipses 1, 2, 3 and 4 of the Bridge are clearly shown.The emission has travelled a distance of 15 light-years in less than five years. The angular scale (2 arc-minutes = 15 light-years at the distance of the galactic centre) and the direction towards Sgr A* are also shown.

This phenomenon proves that these clouds reflect the high energy radiation produced by the intensely active supermassive black hole. The light echo reaching us is delayed relative to the light arriving directly from the black hole, and the length of this delay depends on the position of the cloud and the time taken for the radiation to travel the distance between the black hole and the cloud. By observing the decay of the gamma ray emission from another cloud (Fig. 2), the researchers have been able to estimate the duration and intensity of the flare. It began around four centuries ago and ended at the beginning of the twentieth century. For a period of three hundred years, the supermassive black hole in our Galaxy was therefore a million times brighter than it is today.

Figure 2. The region of the galactic centre as seen by INTEGRAL in low-energy gamma rays (20-60 keV) at different periods, with each strip showing a different year from 2003 to 2009. The various sources are generally associated with variable X-ray binary systems.The green circle in the top image shows the position of the Sgr B2 molecular cloud. The associated gamma-ray source clearly shows decay over 7 years

(Terrier R., et al., 2010, The Astrophysical Journal).

Bibliography

Discovery of a Superluminal Fe K Echo at the Galactic Centre: The Glorious Past of Sgr A* Preserved by Molecular Clouds. http://stacks.iop.org/0004-637X/714/732